What am I missing?
total derivative of unspecified y:
In[1]:= Dt[y,x]
Out[1]= Dt[y, x]
But if we take the derivaitve of an undefined indexed variable
it is assumed to be a constant..
In[2]:= Dt[a[1],x]
Out[2]= 0
note however that the indexed symbol could easily represent a non-constant.
In[3]:= a[1]=y; Dt[a[1],x]
Out[3]= Dt[y, x]
clearly the trouble is that there is no distinction between an indexed
symbol and a function evaluated at a fixed point..
Is there a workaround?